“WASHINGTON — Federal investigators have identified several dozen Pentagon officials and contractors with high-level security clearances who allegedly purchased and downloaded child pornography, including an undisclosed number who used their government computers to obtain the illegal material, according to investigative reports.

The investigations have included employees of the National Security Agency, the National Reconnaissance Office, and the Defense Advanced Research Projects Agency — which deal with some of the most sensitive work in intelligence and defense — among other organizations within the Defense Department.

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Many of those apprehended were swept up in a much broader probe initiated by the Immigration and Customs Enforcement agency in 2006. Operation Flicker identified an estimated 5,000 people who had paid money over the Internet to access websites operated overseas. But until now, it has not been disclosed that a sizable number of cases were referred to the Defense Department for investigation because they involved military personnel, intelligence officials, or defense contractors.

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Specialists in child protection expressed alarm at the revelations, but said it was not that surprising to find even officials in sensitive government positions engaging in such activity. “Some are in high-ranking positions, in positions of trust,’’ said John Sheehan, executive director of the exploited child division at the National Center for Missing and Exploited Children, which has been consulted on many of these cases and has reviewed 36 million images of alleged child pornography since 2002 at the request of law enforcement agencies. “There isn’t a profile or stereotype, which makes it even more challenging for law enforcement.’’ The Pentagon’s investigation reports show that personnel found frequenting the illegal websites worked at a variety of Pentagon installations.

Thirteen suspects were identified in California, including individuals who worked at some of the most sensitive military installations on the West Coast. One was a contractor at Edwards Air Force Base, where weapons testing is conducted, while another worked at the Naval Air Warfare Center at China Lake.

A large amount of pornography was found on the office computer of a program manager at the Defense Advanced Research Projects Agency, including images that appeared to be of children. DARPA is responsible for developing some of the military’s most secret weapons and technologies. Charges were not pursued because there were no images of known victims of abuse, something that is routinely needed to bring charges, the case summary said.”